Third Generation Partnership Project (3GPP) is currently specifying closed-loop transmit diversity (CLTD) features for the uplink (UL) in Rel-11. Uplink transmit diversity (ULTD) schemes use more than one transmit antenna in the User Equipment (UE) to improve the quality of the uplink transmission. The CLTD scheme typically requires the receiver to provide fast feedback information about the spatial channel to assist the transmitter in choosing a transmission format over multiple transmit antennas.
The approach now considered by the 3GPP is that the network (the serving NodeB or the NodeB in the active set pointed out by the Radio Network Controller (RNC) in case no serving cell is configured) decides the precoding vector that shall be used by the UE. A precoding vector includes transmission antenna weights for a set of antennas. The antenna weights are associated with a respective transmit antenna. See also 3GPP Technical Specification 25.212 Multiplexing and channel coding, and 3GPP Technical Specification 25.331 Radio Resource Control (RRC).
There are two ULTD precoding coding schemes “Beamforming” and “Antenna switching.” 3GPP has chosen the ‘Beamforming’ transmitter scheme for the UL CLTD feature. In beamforming, a signal is transmitted from a number of transmit antennas simultaneously with appropriate weighting. Switched antenna diversity implies that one single antenna at a time is used for transmission. In other words, switched antenna diversity implies use of a precoding vector with one transmission antenna weight equal to one, and all remaining transmission antenna weights equal to zero.
There are currently five different UL CLTD configuration modes supported by a UE and the NodeB. In the future, there may be fewer or more different UL CLTD configuration modes. The switching between these modes can be done by dynamic control via Serving NodeB High Speed-Shared Control Channel (HS-SCCH) orders (in case the UE is configured with a serving NodeB). Note further that the switching between at least some of the modes, e.g. CLTD configuration 1 and CLTD configuration 5 can be achieved by synchronised configuration via RNC Radio Resource Control (RRC) reconfiguration messages.
TABLE 1UL CLTD configurationsUL CLTDUplink ChannelsConfigurationDPCCHHS-DPCCHE-DPCCHE-DPDCHS-DPCCH1Primary Precoding VectorSecondaryPrecodingVector2Physical Antenna 1PhysicalAntenna 23Physical Antenna 2PhysicalAntenna 14Physical Antenna 1De-activate5Physical Antenna 2De-activate
The dynamic activation/de-activation of UL CLTD is important because the transmission from more than two transmit antennas will not be beneficial in certain scenarios, and the NodeB can take control of the situation via HS-SCCH orders (using local information that may not be known at the RNC).
There is a constant desire to improve the performance in cellular radio systems. Hence, there exists a need for new methods and devices providing improved performance in cellular radio systems with UEs configurable for UL CTLD.